As a method for improving gas barrier properties of ion-conductive polymeric membranes including a NAFION (registered trademark; available from Du Pont Co.) membrane, Japanese Patent Laid-Open Application No. 2006-327932 discloses a technique in which ion-conductive materials obtained by utilizing silanol groups of inorganic layered structures to bind sulfonic acid groups to the inorganic layered structures are dispersed in an ion-conductive polymeric membrane.
However, in Japanese Patent Laid-Open Application No. 2006-327932, montmorillonite, which has silanol groups only on the end faces, is used as an inorganic layered compound, and hence, as shown in FIG. 1, the sulfonic acid groups are bound with only the end faces of layers 1 formed of the inorganic layered compound. Thus, the ion-conductive polymer composite membrane obtained has insufficient proton conductivity.